Strontium is a relatively rare alkaline-earth metal commonly found as insoluble sulfate, carbonate, silicate and phosphate deposits in the earth's crust. Typically, strontium is recovered from celestite ores which contain strontium sulfate. The strontium sulfate is usually converted to the carbonate form which finds a variety of uses in the electronics and glass industries, particularly glass for use in television tubes where low transmission of x-rays is desired. Other strontium compounds which can be produced from strontium carbonate find uses in the pyrotechnics, chemical, and pharmaceutical industries.
From a practical standpoint, economically producing strontium carbonate of sufficient purity for utilization in the electronics and glass industries requires relatively high grade strontium bearing ore as a starting material. For example, imported celestite ores from Scotland are known to contain up to 98% strontium sulfate. Lower grade ores containing approximately 80-85% strontium sulfate are commonly treated with a prior beneficiation or purifying step such as floatation of the ore to achieve the requisite degree of purity prior to processing.
The most common commercial process for producing strontium carbonate from celestite ore is the "black ash" process in which ground celestite ore is mixed with finely divided carbon and the mixture calcined at elevated temperatures of about 1000.degree. C. to produce strontium sulfide (SrS), carbon dioxide (CO.sub.2) and carbon monoxide (CO). The water soluble strontium sulfide so produced is leached from the black ash calcination product with hot water and the solid impurities are removed by settling and filtration. The dissolved strontium sulfide is converted to strontium carbonate with either sodium carbonate (Na.sub.2 CO.sub.3) or carbon dioxide or both. The strontium carbonate so produced precipitates from the aqueous solution and is recovered and dried using conventional separation techniques.
Though commercially successful, the "black ash" process has disadvantages including the requirement of significantly elevated process temperatures and the disposal of undesirable reactant byproducts such as sodium sulfide (Na.sub.2 S) produced when sodium carbonate is the carbonating compound or hydrogen sulfide (H.sub.2 S) produced when carbon dioxide is the carbonate source.
Alternatively, strontium carbonate can be produced directly in a metathesis reaction by digesting relatively high grade celestite ore with soda ash solutions at elevated temperatures. However, little purification takes place in this process so high purity products cannot be obtained from low or medium grade areas.
These known prior art processes become uneconomical when low or medium grade ores are utilized as starting materials. Low grade ores commonly contain undesirable extraneous materials such as barium, magnesium, calcium, iron and aluminum as well as silicates silicon dioxide and clays. Barium and iron are particularly troublesome extraneous materials because they are difficult to separate from the desired strontium product and hence interfere with the production of high purity strontium carbonate.
Accordingly, one of the objects of the present invention is to provide an economical process for producing substantially pure strontium carbonate from low and medium grade celestite ore. A further object of the present invention is to produce substantially pure strontium carbonate from such ores without the need for elevated temperatures or with the production of undesirable waste products.